A cross sectional configuration of general piston rings installed in a piston of an internal combustion engine is shown in FIG. 9. In a piston 100, ring grooves 110, 120, and 130 are formed for accommodating piston rings. In each ring groove, compression rings 200 and 210 are mainly for gas sealing, and a three-piece oil ring 220 is mainly for oil controlling and oil sealing are inserted respectively. In accordance with the reciprocating motion of the piston 100, outer peripheral sliding surfaces of these rings slide with the inner wall of a cylinder 300.
The three-piece type combined oil ring 220 comprises a pair of side rails 230 and 240 arranged at its axially upper and lower portions, and a spacer expander 250 combined therebetween for pressing the side rails 230 and 240 from inner peripheral side to produce tension on the side rails 230 and 240. Profiles of each of these components are described, for example, in JIS B 8032-13, “Internal combustion engines—Small diameter piston rings—Part 13: Expander/segment oil control rings,” published on Mar. 31, 1999, pages 3 to 5.
Due to the ear portions 252 that are formed with an angle to protrude in the axial direction of the spacer expander 250, the side rails 230 and 240 are pressed with component forces in the radial direction toward the cylinder wall and in the axial direction toward upper or lower surface of the ring groove. Therefore, the side rails 230 and 240 can perform sealing with the cylinder wall and with the upper and lower surfaces of the ring groove 130.
The configuration of such a spacer expander of a combined oil ring is disclosed in JP06-081950A, for example. In this spacer expander, a wave shape is formed in the axial direction by gear machining of a metal strip. The outer peripheral surfaces of the side rails are pressed toward the inner peripheral surface of the cylinder with the tension generated by the wave-shaped portion, and thus secures sealing property. In addition, this wave-shaped portion becomes an inlet and outlet of oil, and effectively prevents sticking. Moreover, in the inner peripheral portion of the spacer expander, protruding portions in the axial direction of the piston (ear portions) are formed for pressing the side rails outward in the radial direction, and protruding portions are formed in the outer peripheral portion for supporting the side rails in the axial direction.
With the recent trend toward better fuel consumption of engines, it has been desired to reduce width and weight of oil rings. As an example of reducing width in the axial direction of a three-piece type combined oil ring, JP04-300467A is reported. The expander of this oil ring is made of a steel strip having a wave shape in the axial direction, in which an upper portion and a lower portion are alternately located and axially spaced apart in a circular arrangement and both portions are connected with an intermediate portion extending in the axial direction. The expander is not inserted between two rails. Tapered faces are formed on both upper and lower sides of the outer peripheral surface of the expander. Inner peripheral surfaces of the upper and lower rails are pushed by the tapered faces so that the rails contact with the inner peripheral surface of the cylinder. Because the expander is not inserted between the upper and lower rails, the width of the oil ring in the axial direction can be reduced.
In JP05-087240A, a spacer expander of a narrow width three-piece oil ring is reported. The spacer expander is made of a shape steel material having a substantially lateral T-shape in cross section. In this spacer expander, side rail supporting surfaces are formed on the top and bottom of an enlarged portion on outer peripheral side, and upper and lower pads for pushing side rails are formed by bending end surfaces on inner peripheral side alternately upward and downward. The pad portions are bent by plastic working, and have thin material thickness and a low stiffness. Thus, the pad portions have a high degree of flexibility in displacement. Therefore, an excellent followability can be obtained in pushing the side rails toward the inner peripheral surface of the cylinder due to the elasticity of the pad portions, and better sealing can be maintained even at low tension, compared to the conventional oil rings.
JP05-106734A discloses a spacer expander of a narrow width three-piece oil ring. Here, the spacer expander is made of a base plate of a shape steel material whose outer peripheral side is thick and inner peripheral side is thin. Slits are formed alternately from the inner peripheral side and the outer peripheral side. The outer peripheral side portion formed between the adjacent slits on the outer peripheral side and the inner peripheral side portion formed between the adjacent slits on the inner peripheral side are bent alternately upward and downward, and the surfaces facing the outer peripheral side support side rails. This spacer expander ensures that the side rails follow irregularities on the inner peripheral surface of the cylinder because the inner peripheral side portion has a low stiffness, and thus easily deforms elastically.
In a combined oil ring, as shown in JP06-081950A, the side rails are combined with the spacer expander so that the inner peripheral surfaces of the side rails contact with ear portions of the spacer expander. The combined width of the oil ring in the axial direction depends on the axial width of the connecting portion that connects the upper portion and the lower portion of the wave shape of the spacer expander. Therefore, if the axial width of the connecting portion of the spacer expander can be reduced, it is possible to reduce the axial width of the combined oil ring when the spacer expander is combined with side rails. However, for conventional spacer expanders, wave shapes have been manufactured by gear machining, and thus it has been difficult to reduce the axial width of the connecting portion due to manufacturing reasons. Therefore, there has been a limit in reducing the combined axial width.
In the configuration of a combined oil ring shown in JP04-300467A, the contact portions of the rails and the expander are only the inner peripheral surfaces of the rails and the tapered faces of the expander. Therefore, it is difficult to control the amount of axial movement of the outer peripheral portion of the upper and lower rails in the cylinder. If the amount of movement of the rail outer peripheral surfaces on the cylinder inner peripheral surface is large, vibration tends to occur between the rail outer peripheral surfaces and the cylinder inner peripheral surface. This may inhibit the reduction of oil consumption by the width reduction, and may cause unusual noises.
In a method that uses a shape steel material as shown in JP05-087240 and JP05-106734A, the steel material itself is expensive in general. Also, the tension of the spacer expander needs to be controlled by the width and number of slits that are formed alternately on the outer peripheral side and the inner peripheral side, and thus, machining process becomes complicated and restricted. In addition, reduction in width and weight is limited due to the thickness and weight of the shape steel material itself.
Therefore, the present invention aims to provide a spacer expander of a combined oil ring with a narrow axial width by simple processes and at a low cost, in which the spacer expander is excellent in followability and stability and effective in improving fuel efficiency of engines.